<!DOCTYPE html>

<html>
    <head>
        <meta charset="UTF-8">
        <title>BFC</title>
        <style type="text/css">
            :root {
                border: 1px solid gray;
                margin: 25px;
            }

            body {
                border: 1px solid blue;
                margin: 25px;
            }

            .wrapper {
                float: left;
                width: 200px;
                height: 200px;
                background: #ddd;
            }
        </style>
    </head>

    <body>
        <div class="wrapper">
            根元素html是BFC
            <br>
            凡是说body是BFC的都是王八蛋
        </div>
    </body>
</html>